South Korea's coastguard denied that divers had entered a capsized ferry on Friday in a bid to locate any survivors from an accident that appears to have claimed the lives of hundreds of passengers, many of them schoolchildren.

YTN television, South Korea's main cable news station, had reported that eight divers were taking it in turns to search the vessel, but the coastguard said that divers had not yet been deployed inside the stricken vessel.

The coastguard also denied television reports that the ship had been fully submerged, over 48 hours after the accident.

Experts say it is now unlikely that any of the 268 missing passengers, many of them children from a school on the outskirts of the capital Seoul, will be found alive.

So far, 28 of the 475 passengers and crew on the Sewol ferry have been officially declared dead and 179 have been rescued.

Rescuers were pumping air into the dining hall of the vessel, where many of the passengers were expected to be.

An investigation into the sinking, South Korea's worst maritime accident in 21 years in terms of potential loss of life, has focused on the role of the 69-year old captain and the crew of the ship.

"He (the captain) may have been off the bridge.. And the person at the helm at the time was the third officer," Park Jae-eok, an official investigating the accident, told a news conference in Mokpo, a city close to the port where rescue operations are being conducted.

It is normal for junior officers to take the helm and the 400 km (300 mile) journey from the mainland port of Incheon to the resort island of Jeju was a regular trip in familiar waters.

The accident happened in calm, shallow waters and investigators have focused on the role of the crew as the vessel appeared to have a clean safety record.

Parents of the missing schoolchildren blamed the ship's captain for the tragedy after he and shipping company officials made emotional apologies for the loss of life.

Witnesses have said that the captain and some of the crew left the vessel while others instructed passengers to remain in place as it began to sink.

Theories about the cause of the accident swirled and investigators declined to comment on reports the vessel had turned before it listed to port and capsized.

Coastguard officials have said the investigation was focused on possible crew negligence, problems with cargo stowage and structural defects of the vessel, although the ship appears to have passed all of its safety and insurance checks.

The captain, Lee Joon-seok, faces criminal investigation, which is standard procedure in South Korea.

Both 69-year-old Lee and the company that owns the ship have apologized for the loss of life, although neither has admitted responsibility.

Relatives were in mourning overnight in a hospital in the city of Mokpo, close to the port city of Jindo, which is acting as a rescue center. Some of them spoke bitterly of the captain.

"How could he tell those young kids to stay there and jump from the sinking ship himself?" said Ham Young-ho, grandfather of 17-year-old Lee Da-woon, one of the dead.

Lee has not made any public statement on whether or why he may have left the vessel before many of the passengers.